The present invention relates to a process for producing thermoplastic-resin particles comprising a styrenic polymer base and a high molecular weight polymer or copolymer polymerized either throughout the base polymer particle or primarily near or at the surface of said base polymer particle. The molecular weight of a thermoplastic will generally dictate the resultant physical properties of the thermoplastic. As a rule, the higher the molecular weight, the higher mechanical properties (tensile, flexural, compressive strength) will be. However, processing of very high molecular weight thermoplastics is quite difficult. For example, the molecular weight (Mw) of commercial polystyrene is 200,000-350,000. This limits the physical properties but ensures good processibility. Bead foams of polystyrene also have Mw's in this range in order to obtain good expandability. However, this again restricts maximizing physical properties.